Among other matters, the IG's report is expected to determine "whether there was sufficient justification under existing guidelines for the FBI to have started an investigation in the first place." Critics of the Trump-collusion probe believe that there was never probable cause that a crime had been committed, therefore, there was no legal basis for launching the investigation.
The findings of the Mueller report– that there was no cooperation or collusion between the Kremlin and the Trump campaign– seem to underscore this broader point and suggest that the fictitious Trump-Russia connection was merely a pretext for spying on the campaign of a Beltway outsider whose political views clashed with those of the foreign policy establishment. In any event, the upcoming release of the Horowitz report will formally end the the first phase of the long-running Russiagate scandal and mark the beginning of Phase 2, in which high-profile officials from the previous administration face criminal prosecution for their role in what looks to be a botched attempt at a coup d'etat.
Here's a brief summary from political analyst, Larry C. Johnson, who previously worked at the CIA and U.S. State Department:
"The evidence is plain–there was a broad, coordinated effort by the Obama Administration, with the help of foreign governments, to target Donald Trump and paint him as a stooge of Russia. The Mueller Report provides irrefutable evidence that the so-called Russian collusion case against Donald Trump was a deliberate fabrication by intelligence and law enforcement organizations in the US and UK and organizations aligned with the Clinton Campaign." ("How US and Foreign Intel Agencies Interfered in a US Election", Larry C. Johnson, Consortium News)